Density functional theory study of water interactions on Mn-doped CeO_2(111) surface

摘要

Spin-polarized density functional theory (DFT+U) periodic calculations have been performed to study water adsorption and dissociation on the 12.5% Mn-doped CeO_2(111) surface. Our results indicated that Mn cation is the surface active site for water adsorption and dissociation reactions. The H_2O molecule preferably adsorbs on a Mn cation, causing some relaxation of the surface O-layer and, thus, facilitating the bonding of one of the H_(H_2O) with the nearest oxygen atom. After overcoming an energy barrier of 0.46 eV, the water molecule could dissociate into OH and H species. The latter configuration is about 50% more exothermic than the molecular one, suggesting the Ce_(0.875)Mn_(0.125)O_(1.9375)(111) surface would be easily hydroxylated under reaction conditions. In addition, the calculations showed that water adsorption on the Mn-doped CeO_2(111) surface did not favor the creation of surface oxygen vacancies as it has been reported for pure CeO_2(111). On the other hand, we created a surface oxygen defect in the slab with structural oxygen vacancies and computed water interactions on the reduced surface. Although, the adsorption of OH species in the O-hole caused many surface and subsurface atomic displacements, no changes in the oxidation state of Mn and Ce cations were detected.